There’s something fascinating about our chromosomes. We have 23 pairs. Chimpanzees and gorillas, our closest living relatives, have 24. If you come to these facts cold, you might think this represented an existential crisis for evolutionary biologists. If we do indeed descend from a common ancestor with great apes, then our ancestors must have lost a pair after our lineage branched off, some six million years ago. How on Earth could we just give up an entire chromosome.

A close look at our genome and the genome of our close relatives reveals that we didn’t. We just combined a couple of them. Every now and then, chromosomes fuse. This fusion occurs as sperm and eggs develop, as pairs of chromosomes fold over each other and swap chunks of DNA. Sometimes two different chromosomes grab onto each other and then fail to separate.

Scientists have observed both humans and mammals with fused chromosomes. Chromosomes typically have distinctive stretches of DNA in their center and at their ends. From time to time, scientists will find an individual that’s short a chromosome, but one of the chromosomes it retains now has an odd structure, with chromosome endings near the middle and other peculiar features.

If humans and apes did indeed share a common ancestor, then it would make sense that two chromosomes fused in our ancestors. The rise of genome sequencing allowed them to test that hypothesis. They found that human chromosome two bears the hallmarks of an ancient chromosome fusion, with remnants of chromosome ends nestled at its core. In 2005, it became possible to test the hypothesis again, when a team of scientists sequenced the chimpanzee genome and could compare it to the human genome. The chimp genome team were able to match human chromosome two to two unfused chromosomes in the chimpanzee genome.

Ken Miller, a biologist at Brown who was an expert witness in the 2005 Dover creationism trial, includes this research in his lectures on evolution. Here’s a video of one of those lectures, where he lays out some of the evidence with impressive clarity.

They were able to do so thanks to the publication earlier this year of the gorilla genome. A comparison of the human, chimpanzee, and gorilla genomes confirms that the ancestors of gorillas branched off from the ancestors of chimpanzees and humans about ten million years ago. Humans and chimpanzees then branched apart later. A comparison between all three species provides a clearer picture of what our chromosomes looked like before they fused, and how they’ve changed since.

Eichler and his colleagues put together a diagram to illustrate this ten-million-year saga, which I’ve adapted here.

By comparing human chromosome two to the unfused versions in the chimpanzees and gorillas, Eichler and his colleagues reconstructed the chromosomes in the common ancestor of all three species:

The bands correspond to segments of each chromosome. The colors represent the two ancestral chromosomes (I’ll just call them green and red to keep from getting bogged down in confusing numbers). The hash marks represent regions of very unstable DNA. These areas, which are full of repeating sequences, are prone to accidentally getting duplicated, expanding the chromosome. They’re also where chromosomes are likely to trade chunks with other chromosomes. That’s why the red chromosome has a little green at the end. It had picked up part of the green chromosome earlier than the common ancestor of us, chimpanzees, and gorillas.

The green chromosome then changed:

Three key events are illustrated here. First, the top of the green chromosome flipped (another common type of mutation, called an inversion). Then a chunk of yet another chromosome got stuck to the end of the green chromosome, marked here in pink. And then a new piece of DNA got stuck at the end of the green chromosome, known as StSat, and marked here as a yellow dot.

The ancestors of gorillas then diverged from the ancestors of chimpanzees and humans. They underwent some ten million years of independent evolution, during which time a lot happened. For one thing, the cap on the green chromosome got duplicated and pasted onto other chromosomes, including the red one, and even on the other end of the green one itself. In the illustration below, the yellow and pink segments, along with the adjoining green segment, are represented by a brown oval:

Meanwhile, the ancestors of chimpanzees and humans were evolving. The two chromosomes continued to change, as shown here.

A copy of StSat got glued to the end of the red chromosome, and then the pink and green segments at the top of the green chromosome got flipped.

The chromosomes at the right of the figure show you what our two chromosomes looked like before they got fused. When the human and chimpanzee lineages split, each lineage inherited them. And in each lineage, they evolved in a different way.

In the chimpanzee lineage, the chromosomes didn’t fuse. Instead, this happened:

The caps on both the green and red chromosomes were duplicated massively and ended up on lots of other chromosomes.

And finally, here’s what happened to humans after our ancestors split from chimpanzees:

The two chromosomes fused, and the cap was deleted, inclusing StSat. It could no longer spread around our genome, the way it did in chimpanzees and gorillas.

This study is an important advance in our understanding of how human chromosomes evolved–a subject of medical significance, too, since the duplication of the DNA at the end of chromosomes can cause dangerous mutations that can cause genetic disorders. Plus, it is very cool to see how our chromosomes are, in fact, an ancient patchwork.

It just so happens that I came across this paper by a very roundabout way–but an instructive one. Over at the Panda’s Thumb, I learned yesterday that biologist Nick Matzke was trying to set things straight on a creationist Facebook page. The page is set up by an outfit called The Biologic Institute, which is promoting a new book by two of its employees that purports to reveal all the flaws in the evolutionary account of human beings. They linked to a post on a site run by the intelligent design clearinghouse, the Discovery Institute, which provided some details from the book. It’s called, “A Veil Is Drawn Over Our Origin as Human Beings,” and it’s written by David Klinghoffer.

Matzke left several comments explaining why they were wrong, and what some of the evidence for human evolution actually is. The Biologic Institute didn’t take this well: they suddenly announced that Facebook was not the appropriate venue for debate, and would limit comments to 100 words, or maybe shut the whole thing down.

I found this deliciously ironic and had to jump in too. I pointed out that the site they linked to does not allow comments (which is fairly typical of creationist web sites). So there was no other way to ask questions than to post them to Facebook. And my question concerned fused chromosomes.

Klinghoffer declares:

The evidence from chromosomal fusion, for one, is strikingly ambiguous. In the Darwinian presentation, the fact that humans possess 23 chromosome pairs and great apes 24 clearly points to an event in which human chromosome 2 formed from a fusion, leaving in its wake the telltale sign of telomeric DNA — normally appearing as a protective cap at the end of the chromosome — in the middle where it doesn’t belong. Ergo, common descent.

But Casey [Luskin, of the Discovery Institute and co-author of the book] explains, there’s a lot wrong with this inference. Even if there was such an event and humans once had 24 chromosome pairs, it doesn’t at all follow that this happened in some prehuman past. Nothing stands in the way of picturing a human population bottleneck accomplishing the spread of a fused chromosome 2 from part of an early human community to all of it.

But the idea of such an event having occurred at all is itself far from sure. The telomeric DNA parked in the middle of chromosome 2 is not a unique phenomenon. Other mammals have it too, across their own genomes. Even if it were unique, there’s much less of it than you would expect from the amalgamation of two telomeres. Finally, it appears in a “degenerate,” “highly diverged” form that should not be the case if the joining happened in the recent past, circa 6 million years ago, as the Darwinian interpretation holds.

I was baffled, so I asked on Facebook for the evidence that the form of the chromosome wasn’t what you’d expect if it fused six million years ago.

What followed was a ridiculous runaround, some of which I’ll reproduce here:

Biologic Institute: Ah! That evidence is in the book that the post describes.

Carl Zimmer: In other words, the only way we can check these claims is to purchase the book? There’s no evidence published in peer-reviewed journals?

[No response]

Carl Zimmer: The book you are pointing us to is written by two Biologic Institute employees–the same Institute that puts out this Facebook page. Why can’t you describe your evidence about the chromosome fusion here?

[An hour passes. No response.]

Carl Zimmer: Hello? Is anyone there? Are you choosing not to respond to my request for evidence from your own book? How do you calculate what the chromosome fusion DNA should look like if it fused six million years ago?

Biologic Institute: Carl, you write books for a living. Do you rehearse their content on your blog for anyone who asks?

Carl Zimmer: Hello, Biologic Institute. If I make a strong claim about science in an online forum, and someone asks me for evidence for that claim, I do not say, “Well, you’ll just have to read my book.” I provide the evidence–I point to the peer-reviewed research on which I based my statement. But, hey, I’d be perfectly satisfied if you pointed me to a scientific paper that presents calculations showing that the chromosome fusion could not have happened six million years ago. I can go find it for myself–if such a paper actually exists.

Well, that was the last I heard from the Biologic Institute. They still haven’t piped back up on their own thread. However, I did hear from someone who had read the book, Paul MacBride. (He even reviewed it here.) Here’s the comment he left on Facebook:

Carl, I can tell you the answer to your question, as I have read the book. Luskin provides no evidence for this. Well, more correctly, he quotes a question from this paper http://www.ncbi.nlm.nih.gov/pubmed/12421751 “If the fusion occurred within the telomeric repeat arrays less than ∼6 Mya, why are the arrays at the fusion site so degenerate?” but not their three suggested answers. Luskin asserts that if a chromosomal fusion occurred it should have been a neat and tidy joining of the two chromosomes in question, anything else is a Problem For Evolution. Dave Wisker addressed this succintly in a comment at Panda’s Thumb http://pandasthumb.org/archives/2012/07/paul-mcbrides-r.html#comment-288503

I read the 2002 paper long ago, but MacBride’s link led me to reread it. I also noticed that it was cited by a number of more recent papers, including Eichler’s new one. It just goes to show how you can end up learning something new in the most unexpected places.

Given that all mammals are proposed to have originated from a common ancestor, and that chromosome number varies from under 20 to over 80, shouldn’t we expect to see hints of ancient chromosomal fusions and breakages in many species? The idea that because other species show evidence of teleomeric sequences within their chromosomes, we shouldn’t think the human one is due to a fusion does not make any sense to me.

[CZ: There is lots of evidence of the evolution of fused and broken chromosomes in other mammals. For example.]

Can I ask an idiot question? Is “chromosome” a real thing, or is it an artifact of human categorization, like “species”? If “chromosome” is just an artifact of the human tendency to divide and categorize, then the argument is moot; but if “chromosome” reflects a material difference in the arrangement of genetic material, then it deserves to be addressed.

I know that the idea of chromosomes predates knowledge of the structure of DNA and genomics, but I’m unclear as to whether it represents material reality or is just a convenient way to describe differences between identified populations.

[CZ: In animals and plants, you can find big packages of DNA that are reliably passed down from one generation to the next. These are chromosomes. They’re quite real, although they do change in structure through evolution.]

You’re funny, Doc Bill. I agree, there is no evidence to support their claims. I sometimes think of all the money being dumped into the Discovery Institute. This could go to work doing things Christians say they love to do, like feed and educate the poor. Instead they use it to print textbooks with nothing but rubbish like the Loch Ness Monster is real, proving that humans and dinosaurs once lived together (in the Garden of Eden).

HP: chromosomes have been known for some time. The reason is, they are big enough to see optically (which is pretty remarkable in itself, being the stored form of microscopic DNA), and techniques were discovered to make them visible.http://en.wikipedia.org/wiki/Karyotype
Chromosomes are indeed complete units, as far as I know; they consist of a long, contiguous DNA strand, twisted up and bound by proteins, which hold the DNA compactly while it’s not in use (something like rewinding old cassette tapes after playing them). The proteins also allow other structures in the cell to grab a hold of the chromosome, which is important when the cell divides, ensuring both cells get a copy of each chromosome.

The key to the karyotype is when a cell divides: a chemical is added which freezes the process, and a dye is added, which binds to the chromosome’s proteins, allowing them to be seen. The individual chromosomes are cataloged by eye, I guess, which sounds all kinds of tedious. In whatever order they were first published in, this is how the human karyotype was first identified.

Since the chromosome is the largest scale structure, it’s useful for tracking gross changes in the genome, even though the genome per se involves the DNA within. It’s the same as saying, you’d probably be able to notice if part of your library of cassette tapes were switched from, say, pop to reggae, without having to listen to each individual tape in the library.

Very nice article – thanks for posting it. I’d just recently learned about the fused #2 chromosome and your summary breakdown helps my understanding. As for your “discussion” with the creationists, your experience highlights two things I’ve noticed with creationists. One, those defending creationism and/or debunking evolution seem to always quote someone else. When you challenge them, the answer is always some excerpt from a book, by an extreme creationist, which then, as you clearly point out, cherry picks from some legitimate source.

The second thing I’ve learned is the base difference between scientists and creationists. Using an analogy of examining a cherry pie to determine how it was made – the scientist tries to break down the components and figure out how they were put together. The creationist brings a recipe with him, and when he discovers there are no cherries in the recipe, he tries to explain away the cherries in the pie.

Thanks to Carl and Tim for your answers. In retrospect, I realize that my “idiot question” may have sounded like a challenge, but it was really motivated by ignorance. Thank you both for clarifying something that’s been fuzzy to me for a long time.

This is priceless! Especially the part about not allowing comments of more than 100 words at Biologic’s Facebook page. They probably will shut the page down, and Klinghoffer or one of the other well-paid DI flacks will write a post at their site (allowing no comments) and complaining about how the Darwinian “establishment” doesn’t take them seriously….

Great article. A Chinese doctor contacted me a few years ago about a man with 44 chromosomes who was perfectly normal but suffering from multiple miscarriages (well, his wife was suffering from the miscarriages). He was found to come from an inbred family that shared a balanced translocation and he got the translocation from both parents. He is an extremely useful case because he shows doubters how a species could go from 46 to 44 (or 48 to 46) chromosomes. I talk about him at http://genetics.thetech.org/original_news/news124.

Most folks I have talked with don’t have trouble with the idea of chromosomes fusing but instead are troubled about how it might spread through a population to become the dominant chromosome number. We talk about that at http://genetics.thetech.org/ask/ask229.

“Scientists have observed both humans and mammals with fused chromosomes.”

“If humans and apes did indeed share a common ancestor…”

This may be considered overly critical of me, but doesn’t this use of terminology contribute to further confusion with this entire issue?

There are good reasons to use these words. In medicine human and animal are used to allow for differentiation of specialized fields, Veterinary and Human medicine. They are not statements of decent in this case, only words that speak of which side of the medical field one practices on. There is nothing scientific about the use of these words in medicine. Unfortunately, misconceptions about this have cased widespread misuse and exacerbated the confusion.

I understand it is easy to lapse into poor usage of these words. But I think it is important that we maintain correct usage. I have had too many experiences where students have long held misconceptions about issues ranging from what is a theory verses a hypothesis, to humans are not animals, because an educator used these terminologies incorrectly.

I know what I mean when I say human and animal, but whenever I have to talk about it I always use humans and non-human animal, or human and non-human apes. Just like I would say avian and non-avian dinosaurs.

I don’t mean to pick on your article, the human chromosomal 2 fusion is a wonderful piece of our evolutionary history, and your work here to make more people aware of it is very respectful.

CZ, if I recall correctly, this very discussion (the number of chromosome pairs between humans and apes) was a piece of evidence used in Kitzmiller v Dover. If so, I find it absolutely delicious that it was brought out *again* to cause those who would misinform and mislead to scurry back to their hidey-holes. Well done!

[CZ: Indeed, here is the transcript of the day Ken Miller described chromosome fusion during his testimony.]

As a Christian I get really annoyed by these so-called “Creationists” and their attempts to “prove” the creation poem in Genesis is the “real” story. They get so hung up on the word “day” (MISTRANSLATION FROM THE HEBREW! It is actually “era” but can also be “epoch”) they make idiots of themselves and paint all Christians as idiots. It is morons like these that give all people of faith a bad name and fuel the myth that mainstream Christianity, as opposed to the fundamentalist variety found in these literalist enclaves, oppose Science and Evolution as a theory. They don’t, that was the selective propaganda of Huxley and others in the late 19th and early 20th Century, their misquotes and contorted reporting did neither science nor religion any favours.

Darwin himself was concerned at a number of missing links in his theory and this was one reason he delayed publication. The bishop often misquoted as pouring scorn on his theory – if anyone bothers to read what was actually said – was in fact telling Darwin he needed to show how these gaps could be closed. Now, thanks to the work on the genome project (to which I have happily supplied a sample of my DNA) we are beginning to understand how and when some of these divergences have occured. I wonder if the “DIscovery Institute” is aware of the fact that all humans outside of Africa belong to seven major Haplo Groups, while those in Africa belong to any one of up to 45? Perhaps they’ll resort to Eugenics next to explain that – or maybe Noah?

They also miss the fact that there are two “Creation” stories in Genesis. Chapters 1 and 2 are not continuations, but two seperate stories. In short, if read without bias and in the knowledge of what someone a little over six thousand years ago came up with without knowing about evolution or the “Big Bang” theory, comes pretty close to providing a simplistic explanation of how everything came into being – provided one reads that infernal word “day” in its correct interpretation of “era” or “epoch.” Either of those terms can and does usually mean anything from a year to several million years. Not 24 hours.

Thank you Mr. Zimmer for your excellent explanation. The vast majority of Christians certainly do not share this fixation with literal interpretations of the Bible.

So here’s a question: Would that kind of fusing have caused a massive bottleneck-type impact on the community? Here’s the track my brain is following (bear with me…I’m realizing how much of this stuff I don’t remember from high school bio): If one lady has a fused chromosome 2, and then mates with an unfused partner, since the mate now has an ‘extra’ chromosome, a bunch of his DNA would not be linked to anything during fertilization? In other words, you’re trying to match up 24 chromosomes to 23, so someone loses musical chairs and gets tossed out of the game. This means the kid only gets a large chunk of Chromosome 2 from one parent, so we’ve lost some diversity. And this happens over and over again until that one parent’s chunk of Chromosome 2 becomes the only game in town.

Or would the unfused partner’s extra chromosome recognize the telomeres (at least while they were still there) and fuse correctly to Chromosome 2 anyway?

Chromosome fusions are common in some species, like house mice. The ‘standard’ mouse has 40 chromsomes (20 pairs), but thay can go all the way down to 22 chromosomes (11 pairs) where every single one of the chromosomes have buddied up with another chromosome (apart from the sex chromosomes, which can’t).

Re: Megan. Good question. If you imagine an individual has a recently fused 2:5 (chromsome 2 and 5 have joined), when they breed with a ‘standard’ chromsome set, the unfused 2 pairs up with the 2 part of the 2:5, the unfused 5 pairs up with the 5 of 2:5 and meiosis continues as normal. Surprisingly, this often doesn’t lead to much loss of fertility (in mice. I don’t know about humans). So Chromosome 2 contiues its merry way into the next generation unimpeded.

Where it gets silly is where different chromosomal races meet and breed. If they have different fused chromosomes, these have to try to match up and can lead to long chains of matched up chromsomes or even rings. Unsurprisingly, they do suffer a loss of fertility…

‘Tis a great post Carl, in which you’ve used most effectively, illustrations to demonstrate the molecular evolution of these chromosomes within the Great Apes (including of course, humans) these past ten million years.

What scares me about creationists and their muslim cousins is that they have the money and moxie to promote their non-sense!

With the oh-so-right/christian/creationist you tend to get Inquisitions and with Muslims you get primitive executions. It’s in the BOOK! Praise God! I guess it dosen’t matter how you go as long as you go away.

So, might we be contacted by our makers/creators if we showed even a tiny bit of sanity, intelligence and humanity? A different looking thing-r-ma-other saying “Hi!” ” Welcome to the Community!”

Mean while a small group of scientists study Earth and Life and we slowly get to the truth.

If one lady has a fused chromosome 2, and then mates with an unfused partner, since the mate now has an ‘extra’ chromosome, a bunch of his DNA would not be linked to anything during fertilization? In other words, you’re trying to match up 24 chromosomes to 23, so someone loses musical chairs and gets tossed out of the game. This means the kid only gets a large chunk of Chromosome 2 from one parent, so we’ve lost some diversity. And this happens over and over again until that one parent’s chunk of Chromosome 2 becomes the only game in town.

Actually the problems only really appear in the next generation.

If we assume our lady here as a fused chromosome AB, while her mate has unfused chromosomes A and B, her egg will have one AB (she herself of course has two copies AB), and her mate’s sperm will have one A and one B. The resulting child gets a diploid chromosome set of [AB, A, B] (compared to mom who has [AB, AB] and dad who has [A, A, B, B].

Now when this child grows up and tries to make gametes, he or she will have an odd, rather than even number of chromosomes to divide up, and what generally happens is that all possible divisions are made, more or less at random. So sometimes the gametes will be divided [AB] amd [A, B], sometimes [AB, A] and [B], sometimes [AB, B] and [A].

The gametes that are [AB] and [A, B] are basically normal, and if the child mates with either a fused or unfused partner, will have no problem. But gametes with [AB, A] and [AB, B] will have an extra chromosome if mating with a “normal” partner while those with just [B], [A], will have a chromosome missing, if mating with a “normal” partner. These result in zygotes that may not be viable due to the genetic imbalance with either excess or missing genes.

And that means that since the distributions of chromosomes in the gametes are more or less made randomly, a proportion of the gametes made will be “no good”, and this may result in reduced fertility.

Saint Duffus, creationists can be Fundamentalist Christians, Fundamentalist Jews AND Fundamentalist Muslims, among others. I wouldn’t distinguish creationists from “their muslim cousins” when they are merely different, but related, branches of the same philosophical-religious “phylogenetic tree”.

The situation of the grandchild above is also the situation when the fusion mutation first appears. So the first individual with the fusion, born from two unfused parents, will probably be [AB, A, B], because it is rather unlikely that the same fusion mutation turning A, B into AB would have just happened to have occurred simultaneously in BOTH the specific egg and sperm that produced the new individual.

So this individual more or less will end up with 1/6 (give or take, the distribution is not necessarily purely balanced randomness in real life) of gametes as AB, and these gametes, if mated with a normal unfused individual, are perfectly viable, and will produce offspring with [AB, A, B] just like the fused parent.

Another 1/6 of gametes will be [A, B], and these are basically completely normal, “old-type” gametes and will have no problem mating with unfused individuals.

The other 4/6 gametes may or may not produce viable offspring. Let’s assume the worst case scenario and say they are all lethal, and ignore them.

Now if our original fused mutant produces only a small amount of gametes (like say a human woman, producing roughly one egg per month), then having 2/3rds of her gametes basically be broken can significantly impact her fertility. But she still has a 1/3 chance of successfully reproducing, and a 1/6 chance of having a viable child share her own fused mutation.

If our original mutant is one who produces lots and lots of gametes (like say a human male, producing trillions of sperm over his lifetime), the fact that 2/3rds of the gametes are broken may nor necessarily impact his fertility much at all, as the 1/3rd “good” gametes are still enough to get the job done. And again, half of his viable offspring will carry his fused mutation.

Thus, by genetic drift, the fused mutant genotype will spread into the population, usually at a low number.

Now, eventually, it may come to pass that two fused mutants will mate, and if it just so happens that they both pass the fused AB to one of their children, that child becomes a new double fused mutant, with [AB,AB]. So [AB, AB] double mutants will slowly spread through the population by genetic drift as well. And if two [AB, AB] individuals eventually meet and mate, their children will inherit the [AB, AB] double mutation reliably.

And if at some later date some event occurs (and it could really be anything at all) that eliminates the old [A, A, B, B] phenotype from the population, then you’ve suddenly got a population that has evolved from pure [A, A, B, B] to pure [AB, AB].

And this can all happen solely from genetic drift. Natural selection does not have to play any role at all (although it can).

There is also the posibility that loss of A or B in the gametes generate inviable gametes, in which only those with AB survive… or at least, the AB scenario may have a selective advantage in terms of gamete survival. If I’m thinking about this correctly, then a much larger percentage of the gametes will be viable… does anyone know if non-human simians can loose one of the chromosomes and still survive?

@ The Gray Monk
If you actually read Genesis in its original language you find the word used is yom which is used in every way as denoting either a 24 hour period, or as the 12 hours of sunlight during the day. Yom Kippur for exapmle is the Day of Atonement.

Your question betrays a typical creationist misunderstanding of this issue.

It is not claimed that the fusion was a crucial aspect of the evolutionary divergence of humans from other primates.

The point is that, once the fact that humans possess one fewer chromosome pair than other primates was discovered, evolutionary theory allowed the prediction that this had resulted from a fusion in the human genome. And, much later, this prediction was confirmed. That’s the ultimate test of the validity of a scientific theory: It’s ability to predict future observations.

So how would ID creationism have allowed this prediction to be made? Inquiring minds want to know.

As I explain in the post, we can trace the origin of the fused chromosome by comparing it to those of chimpanzees and gorillas.

The alleged fusion has nothing to do with chimps and apes. It occurred in the human line only.

What part of that don’t you understand?

Where are the humans without the fusion?

This only makes sense if we share a common ancestor.

Or a common design.

[CZ: In response to your question, “Where are the humans without the fusion?”–All living humans have this particular chromosome fusion. The best explanation for this is that this fusion arose and spread throughout the entire population before the time of the last common ancestor of all living humans. If this fusion occurred, for example, in in our ancestors four million years ago, you’d expect this pattern. This explanation is entirely consistent with what we know about biology: chromosome fusions arise every now and then, some of them have no negative effect on health or reproduction, and such mutations are able to spread throughout an entire population over the course of generations.

That is why there are no humans without the fusion–because no one (that we know of) has experienced a new mutation that broke chromosome two apart at precisely the juncture where the original chromosomes fused.

As for your claim that the fusion has nothing to do with chimps and apes–I assume you mean chimpanzees and gorillas? (Chimps are apes.) Looking at chimpanzees and gorillas tells us what the original chromosomes were that fused in our own lineage. We can see, segment by segment, the match between our own chromosome two and the corresponding chromosomes in our chimpanzee and gorilla relatives. So saying the fusion has nothing to do with chimps and apes is a bit like saying that I have nothing to do with my cousins.

As for this making sense from a common design, you’ll have to explain that one to me. If a fused chromosome in our ancestors is evidence of design, then you’ll have to explain the new design that explains each and every chromosome fusion found today in living humans–including the ones that cause Downs Syndrome and infertility. Good luck with that.]

Sorry Joe G, based on our most robust biological data (genomic, comparative anatomical) and the fossil record, humans can be viewed as part of the clade comprising the Great Apes. Your citation of pseudoscientific creationist nonsense like that link you provided is utterly ridiculous. (If I as someone not really trained in molecular biology or in mammalian systematics can understand this, then any genuinely scientifically literate person should understand this as well.)

Sorry John Kwok- common design is based on our most robust biological data and the fossil record. And a clade is an example of a common design and all its variations.

Your pseudoscience beliefs- common ancestry has to be assumed and cannot be tested- mean nothing to me. You don’t even know what makes a human a human nor a chimp a chimp so you don’t know if one can evolve into the other. All you have are slight changes plus eons of time- pseudoscience.

But anyway- more for you to avoid:

What is the testable hypothesis that the alleged fusion was a random genetic change?

What is the testable hypothesis for how the alleged fusion became fixed?

And finally, what happened to the organisms with a mixture of chromosomes 23/24? They must have existed. The case for a chimp-lineage fusion would be bolstered by such a population.

It is not claimed that the fusion was a crucial aspect of the evolutionary divergence of humans from other primates.

I never claimed otherwise, so what’s your point?

The point is that, once the fact that humans possess one fewer chromosome pair than other primates was discovered, evolutionary theory allowed the prediction that this had resulted from a fusion in the human genome.

In the HUMAN genome- meaning it had nothing to do with any alleged common ancestry. Also what “evolutionary theory” are you referring to? The theory of evolution didn’t even predict chimps nor chromosomes.

So how would ID creationism have allowed this prediction to be made?

Well ID creationism only exists in the minds of the wilfully ignorant so I doubt it makes any predictions. But anyway I posted a link to an explanation that does not require common ancestry with chimps- it is by someone who holds a PhD in cell biology, which means he is more qualified than Carl to discuss this.

Joe is right that the chromosome fusion isn’t really good evidence for common descent. We know that human and chimp/gorilla chromosomes are very similar. For the people who believe in special creation, they just accept the similarity and say that humans were created with the same chromosome sets as apes. The fusion then occurs in humans alone. It does imply that humans have been around long enough for the fusion to come to fixation in the population, and since it is probably a neutral change, that implies that people have been around for a long time, unless fixation occurred in a small early population.

Much better evidence for common descent is the millions of complex mutations that have left their marks at exactly the same locations in ape and human genomes, especially the 3 million transposition events that have left transposon relics at the corresponding locations in primate genomes. Accounting for these without common descent requires that millions of events, with the specific details of each (truncations, deletions, transductions of adjoining sequence, rearrangements, etc.), were repeated exactly in parallel in each of multiple species, and that the pattern of presence/absence of each event in primate species matches the pattern of branching expected from common descent. Also, in many cases transposons have inserted in preexisting transposons so it is possible not only to determine which insertions are present in each species, but what order they occurred in. It all matches the pattern expected from common descent, and is staggeringly unlikely otherwise.

[CZ: They’re not just similar–you can see how they got to their current state through evolution from a common ancestor. But it is true that they’re just a tiny fraction of all the molecular evidence for common descent.]

That’s funny, Joe. The link you provided goes to a story written by a self-described creationist. He looks at Chromosome 2 from two perspectives: evolutionary and ID (in his own words). How is this not ID creationism? 🙂

“But anyway I posted a link to an explanation that does not require common ancestry with chimps- it is by someone who holds a PhD in cell biology, which means he is more qualified than Carl to discuss this.”

Joe, if Carl Zimmer is under-qualified to discuss this stuff – where on earth does that leave you?

“Sorry John Kwok- common design is based on our most robust biological data and the fossil record. And a clade is an example of a common design and all its variations.”

Common design isn’t supported by such data, including the fossil record, Joe. Instead, they imply “descent with modification”. (Darwin’s original term for biological evolution)

“Your pseudoscience beliefs- common ancestry has to be assumed and cannot be tested- mean nothing to me. You don’t even know what makes a human a human nor a chimp a chimp so you don’t know if one can evolve into the other. All you have are slight changes plus eons of time- pseudoscience.”

While I will admit that common ancestry does not have to be assumed, the evidence that exists is so overwhelming that only a fool or someone suffering from some acute delusion would refuse to admit this. Moreover, what makes you think that I don’t know “what makes a human…… a chimp”? To leap to that conclusion is an exercise in abysmal logic.

As for Carl Zimmer’s qualifications to discuss biological research as significant as the latest on human chromosomal fusion, he’s demonstrated that ample times. You have yet to demonstrate once that you have any sound understanding of science, especially when you link to an article written by a Bible thumping Intelligent Design creationist. Methinks you need to read books written by Carl, Sean B. Carroll, Paul R. Gross and Barbara Forrest, among others, before commenting further here.

I didn’t ask for some ridiculous pseudoscientific drivel that attempts to explain the chromsome 2 fusion after it had already been discovered. I asked for an explanation for why evolutionary biologists were able to correctly predict the existence of this fusion before the technology even existed to demonstrate it. If it was the creationists who correctly understood human origins, we would be expecting them to be the ones making such predictions, wouldn’t we?

In response to your question, “Where are the humans without the fusion?”–All living humans have this particular chromosome fusion. The best explanation for this is that this fusion arose and spread throughout the entire population before the time of the last common ancestor of all living humans.

What was so advantageous about this fusion that allowed for it to become fixed? How can we test the claim that the change was random and then how it became fixed?

If this fusion occurred, for example, in in our ancestors four million years ago, you’d expect this pattern.

Nonsense. We would expect to see humans with 48 and some with 47

This explanation is entirely consistent with what we know about biology: chromosome fusions arise every now and then, some of them have no negative effect on health or reproduction, and such mutations are able to spread throughout an entire population over the course of generations.

Only by design or via some severe bottle-neck.

That is why there are no humans without the fusion–because no one (that we know of) has experienced a new mutation that broke chromosome two apart at precisely the juncture where the original chromosomes fused.

No need to break it. There were humans without the fusion. There must have been humans with 47 chromosomes too.

As for your claim that the fusion has nothing to do with chimps and apes–I assume you mean chimpanzees and gorillas? (Chimps are apes.)

It has nothing to do with our alleged common ancestry with chimps.

Looking at chimpanzees and gorillas tells us what the original chromosomes were that fused in our own lineage.

In OUR lineage, meaning it does not have anything to do with common ancestry with chimps.

We can see, segment by segment, the match between our own chromosome two and the corresponding chromosomes in our chimpanzee and gorilla relatives. So saying the fusion has nothing to do with chimps and apes is a bit like saying that I have nothing to do with my cousins.

Common design explains similarity.

As for this making sense from a common design, you’ll have to explain that one to me.

Common design explains the similarity Carl. Houses that are built to building codes will have some degree of similarity due to those codes. Cars follow a common design, as do computers and their peripheral devices.

If a fused chromosome in our ancestors is evidence of design,

No, common design explains similarities, not the fusion. The fusion could have occurred by design, ie one step that isolates us from reproducing with chmps.

then you’ll have to explain the new design that explains each and every chromosome fusion found today in living humans–including the ones that cause Downs Syndrome and infertility. Good luck with that.

[CZ: Mutations do not need to provide selective advantage to become fixed. It’s called genetic drift. I don’t understand your claim that we would expect to see some people with 48 chromosomes and some with 47. This trait went to fixation hundreds of thousands of years ago, if not millions. You also claim that it could only happen by design or a bottleneck with no explanation. Genetic drift works fastest in small populations, but it can happen in larger ones, too. As for design, the only arguments I’ve seen for them is that traits must have been designed because there was no way for them to have evolved or because they show evidence of a plan to serve some function. All the evidence scientists have published about this chromosome are fully consistent with evolution through mutation, drift, and other well-documented natural processes. And you have no suggestion to offer as to what purpose this fusion serves.]

My evidence for my statement is the total lack of evidence in the peer-reviewed literature.

Then there is the Durrett and Schmidt paper on waiting for two mutations.

[CZ: Thank you for providing a reference to a paper! You have far surpassed the people at the Biologic Institute and Discovery Institute on this score. By pointing me to a paper, I can now evaluate your evidence to see if it justifies your claim. And it does not. Don’t take it from me. Take it from Durrett and Schmidt themselves, who wrote a letter to rebut the claim of creationists that their paper somehow undermined evolution.]

There is no need to cake a case, Joe. It has been already made by creationists themselves. Henry Morris, the late head of ICR, pointed out that all ID arguments are warmed-up creationist stuff. Bacterial flagellum? Check. Specified complexity? Check. Universal probability bound? Check.

It’s your turn, Joe, to tell us what new arguments the ID “scholars” came up with.

I echo olegt’s request. You do need to tell us what new – and convincing – arguments Intelligent Design “scientists” claim that they have uncovered, but have not yet published in peer-reviewed scientific journals. If their arguments were so persuasive, they would have made it into credible science textbooks like Miller and Levine a long, long time ago. They had more than twenty years to propose such arguments, but haven’t done so yet. How come Joe G? What’s taking them so long? Surely you can’t join them in accusing “Darwinists” of “persecution” or being “EXPELLED” since some of them, like Michael Behe and Scott Minnich, have had peer-reviewed research, not pertaining at all to Intelligent Design, published in scientific journals.

“My evidence for my statement is the total lack of evidence in the peer-reviewed literature. ”

Your evidence is a lack of evidence? Presumably you mean a lack of evidence to the contrary?

In that case, you be interested in buying my rocket engine that runs on sand. You don’t have to check it works, because there is absolutely no evidence in the peer-reviewed literature that says they don’t work.

@CZ
Thanks for the link. I wonder why the Creationists you mentioned didn’t bother doing that kind of research before making their claims
@JoeG
“The alleged fusion was not a prediction of the theory of evolution.”

In what sense? Once we had established that humans and chimps shared a common ancestor with other great apes, and that humans had one fewer pair than the other animals, then the most likely explanaition is that there was a fusion in the human line after it split from the chimp one. CZ even mentions this in his article. It was absolutely a prediction of the theory of evolution.

Hey joe g, if ID isn’t creationism then are you claiming that the designer is not the creator? If the designer and the creator are separate, does the designer work with/for the creator or are they rivals? If they work together, does the designer submit its designs to the creator and then the creator creates humans and other things based on those designs? If there is only a designer and no creator then how does the designer implement its designs?

Is there only one designer? Is there only one creator? Who or what designed and created the designer(s)? Who or what designed and created the creator(s)? When, where, and how did all the alleged designing take place? What, in nature, is designed and what is not (five of each for now), and how do you determine that?

The whole truth wrote “Hey joe g, if ID isn’t creationism then are you claiming that the designer is not the creator? If the designer and the creator are separate, does the designer work with/for the creator or are they rivals?”

And to my knowledge, no ID proponent has yet addressed the manufacturing problem except for William Dembski, who once opined (in Intelligent Design Coming Clean) that a designer could “impart” (lovely weasel word, that) information into the matter and energy universe via a zero-energy, zero-channel capacity link. Oh, and one mustn’t forget Michael Behe’s puff of smoke conjecture.

Given that gene sequence along a chromosome can be altered, often without changing phenotype, the very existance of homologous regions of chromosomes (regions with the “same” genes in the same sequence) in humans and any other organism (and there are plenty, known generically as shared syteny: http://en.wikipedia.org/wiki/Synteny) is strong evidence against special creation, unless god wanted to be purposely deceptive, which would not be very nice.

This is a very good thread, no wonder both Richard Dawkins and Jerry Coyne referred here. That joe g character is very interesting, hope he comes by more often. It is good to know how the specimen works, in order to understand the symptoms of the diseased minds .. Brave CZ! you don’t have to be doctor Zimmer, only IDers make issue of that.

If the fused chromosome 2 also occurs in Neanderthals, this would demolish the notion that the event occurred in humans and not the common ancestor of humans and Neanderthals. The likelihood of a fusion event in the same place in two subspecies is infinitesimal.

For all his ignorance, Joe G does have half a point: The chromosome 2 fusion does little by itself to support all the tenets of evolutionary theory. It mainly supports the existence of common ancestry between humans and other apes.

Given that, it’s a bit puzzling why the ID crowd are making such an issue of this. They repeatedly go to great lengths to claim that they are not creationists. Oh, no, not at all. Creationism is a religious idea, whereas ID is a scientific theory, right? And ID is not supposed to have any issue with the idea of common ancestry. It’s only the religion-based creationists who are all icked out by the idea that we might have apes in our family tree . The ID crowd, being actual for-real scientists, with real degrees and everything, are beyond allowing their conclusions to be motivated by such religious and emotional issues. So they say.

So accepting the idea of common ancestry should not be a problem for them. After all, they don’t need to claim that God – Sorry! The “Intelligent Designer” – did any more than reach down and tickle our genomes just enough to cause the mutations required for us to become human. And all the evidence of common descent would still exist if that happened. No need to believe that humans were specially created by magic all on their own, like the Bible says.

So I’m a bit confused. If Joe G. and the rest of the ID brigade are correct in claiming they are not creationists, then why do they object to the idea of common descent, as one would expect only a creationist to do?

They refuse to acknowledge that they’re creationists simply because it would remind others that creationism has been shown to be religiously-inspired pseudoscientific nonsense by scientists, lawyers and judges. Moreover, they claim that Intelligent Design is “scientific” since it doesn’t specify the Creator’s identity. However, if you’ve seen any clips of Behe, Dembski and their fellow Discovery Institute IDiots lecturing before church audiences, you’ll hear them admitting that the Intelligent Designer is none other than the Judeo-Christian (and Muslim) GOD.

Thanks, John Kwok. I’m not as naive as my post may make me appear. I’m well aware of the mendacity of the creationist movement, and the fact that, in addition to the factors you mention, they adopted the term “intelligent design” after the US Supreme Court confirmed that creationism was a religious idea and therefore cannot be taught in public schools without violating the First Amendment. My main point is that, try as they might, the IDiots can’t help exposing themselves as garden variety, fanatically religious creationists. Taking the claims of ID on their own terms, they have no reason to dispute the chromosome 2 fusion as evidence of common ancestry. They just keep forgetting that they’re supposed to keep up the ruse that they’re not creationists.

Which is why if they were really serious about Intelligent Design as a better, more comprehensive, scientific theory than current evolutionary theory, they would have tried to come up with an Intelligent Design explanation for chromosomal fusion. But they can’t since Intelligent Design isn’t science, but instead, religiously motivated pseudoscientific nonsense. In other words, they can’t have their cake and eat it too.

I knew already, based on your prior commentary with regards to Joe G’s breathtaking inanity, that you’re “not naive”. Thanks again for stating what should be obvious to Intelligent Design advocates regarding chromosomal fusion in humans. Since we have sequenced a substantial portion of the Neandertal genome, it will be interesting to see whether they, too, share the same chromosomal fusion. If so, then one could infer that this was a trait that arose early on in the history of hominids.

It would indeed be interesting to find out whether the Neanderthals have the fusion. (I’m assuming there is enough uncertainty about the date of its occurence that we do not know whether it occurred before the divergence between us and them. I could easily be wrong on that.)

If it turns out Neanderthals do have the fusion, I wonder how the creationists will respond to that. “Common designer”, I would guess.

As I inferred in comment #70, what are the odds that the fusion, if it is found in the Neanderthal DNA (and I’m willing to bet that it does), took place at the same location in Homo sapiens sapiens and Homo sapiens neanderthalis? Probably a lot longer then Dumbski’s specified complexity odds.

Absolutely, I am in agreement. If the same chromosomal fusion occurs in the Neandertal genome, then it would probably give us a minimum time of somewhere around 300 to 400 hundred thousand years for that to have occurred, anda maximum sometime between 7 and 5 million years.

“….then you’ll have to explain the new design that explains each and every chromosome fusion found today in living humans–including the ones that cause Downs Syndrome and infertility. Good luck with that.”

I already responded to the first part so now I’ll respond to the second part, the part about random mutations messing up the design.

First of all, your imaginary designer-god must be a total klutz if it didn’t design things so that random mutations are prevented from messing up its designs. Maybe you could explain how a designer-god that allegedly designed and created the entire universe and all living things could be so inept? I suppose you could say that the designer-god isn’t inept but then you’d need to explain why the designer-god of the universe and living things would be so cruel as to deliberately design destructive/deadly mutations into living things and to deliberately design catastrophes that ‘mess up’ living things or the rest of its designs.

And by the way joe, you seem to have a problem making up your mind about whether mutations are or can be “random”. For instance, here you are posting as “ID guy”:

ID guy Says:
October 23rd, 2009 at 7:30 am

(don provan): I’m not objecting to the observation that bacteria have some control over their mutations. I’m just pointing out that, despite that, there is a random element to mutations.

(joe): Yes I know there is a random element- not all mutations are directed.

All you have done, if anything, is show that some mutations are random- and that is because the researchers refuse to accept that organisms are designed.

Ya see peter, the only way to say that all mutations are random in any sense of the word is to demonstrate stochastic processes can produce living organisms- IOW just as I have been telling you the origins is essential to understanding any subsequent evolution.

(Zachriel): The variations occur regardless of whether they increase fitness.

(joe): The variation ensures fitness.

(Zachriel): I’ve provided you a link to a simplified description of the Lederberg Experiment. I’ve discussed it with you in some detail. The biological community considers it a valid demonstration of random mutation.

(joe): What biological community is that and how did they determine the mutations were “random”- ie not directed?

So, joe says “Only ignorance sez genetic variation is “random” in any sense of the word.”, but he also says “Yes I know there is a random element- not all mutations are directed.”

joe, will you please state your opinion as to whether mutations (genetic variations) are or can be random, and how you’ve determined which mutations are random (if any) and which ones are directed by ‘the designer’?

interesting piece. what i am missing here (and it’s probably my ignorance) is what happens next after chromosomes are fused in one individual. it was my understanding that to procreate sucessfully, both parents need to have matching chromosome sets?

“We have 23 pairs. Chimpanzees and gorillas, our closest living relatives, have 24. If you come to these facts cold, you might think this represented an existential crisis for evolutionary biologists. If we do indeed descend from a common ancestor with great apes, then our ancestors must have lost a pair after our lineage branched off, some six million years ago. How on Earth could we just give up an entire chromosome.”

Well couldn’t you just as well say the most recent common ancestor was a creature with 23 pairs and the chimps and gorillas evolved a 24th? It wouldn’t contradict the theory of Evolution. You would just have to say chimps and gorillas both developed a 24th. It would be convergent evolution. It doesn’t seem like a crisis Evolution had to overcome with humans having 23. Where were the statements from Evolutionary scientists saying Evolution has a potential crisis on its hands when the testing wasn’t done yet to resolve the problem?

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